OSX sublimetext.app v3047:
Drag and drop a folder from the Finder, into the Folders panel, or use Project | Add Folder to Project
Note that no new files are listed in the folder panel, just the folder name
Use the menu item : Project | Refresh Folders
Note that no files are listed in the added folder, still
Quit & restart ST
Now the files are listed in the Folders panel

edit: I cannot seem to replicate this bug any more. I guess it was a transient problem or something